SEOUL, Korea, Republic Of - North Korean leader Kim Jong Un ordered his military on standby for nuclear strikes at any time, state media reported Friday, an escalation in rhetoric targeting rivals Seoul and Washington that may not yet reflect the country's actual nuclear capacity.

The threats are part of the authoritarian government's ramped-up propaganda push to signal strength at home and abroad in the face of what it portrays as an effort by South Korea and the United States to overthrow its leadership.

In a show of anger over the recent adoption of harsh UN sanctions over its recent nuclear test and long-range rocket launch, the North fired off short-range projectiles into the sea Thursday. Friday's report also comes ahead of huge U.S.-South Korean war games set to start next week that the North claims are invasion preparations, and amid a much harder line from rival Seoul meant to squeeze Pyongyang.

"The only way for defending the sovereignty of our nation and its right to existence under the present extreme situation is to bolster up nuclear force both in quality and quantity," a dispatch from the North's official Korean Central News Agency said, paraphrasing Kim. It said Kim stressed "the need to get the nuclear warheads deployed for national defence always on standby so as to be fired any moment."

North Korea has threatened nuclear war in the past, but it is unclear just how advanced the country's nuclear program really is. Pyongyang is thought to have a handful of likely crude atomic bombs, but there is considerable outside debate about the state of its arsenal.

Most experts say it's highly unlikely that the North currently has a reliable, intercontinental ballistic missile capable of reaching U.S. shores, let alone the ability to arm it with a miniaturized nuclear warhead. But the North can probably place nuclear warheads on its shorter-range Scuds and its 1,300-kilometre-range Rodong missiles, which can strike targets in South Korea and Japan, said Lee Choon Geun, an analyst from the South's state-funded Science and Technology Policy Institute. Other analysts, however, question this.

Kim made the most recent warning while guiding the test-firing of a new large-calibre multiple launch rocket system, the report said.

The North's report didn't say when the test-firing happened, but many in South Korea believe it likely refers to the six short-range projectiles that Seoul said North Korea fired into the sea on Thursday.

South Korea's Defence Ministry said the North Korean projectiles, fired from the eastern coastal town of Wonsan, flew about 100 to 150 kilometres. Ministry officials said they couldn't confirm whether the projectiles were those fired by the weapons system KCNA referred to.

Thursday's firings were seen as a "low-level" response to the UN sanctions, with North Korea unlikely to launch any major provocation until its landmark ruling Workers' Party convention in May, according to Yang Moo-jin, a professor at the University of North Korean Studies in Seoul.

The UN sanctions include mandatory inspections of cargo leaving and entering North Korea by land, sea or air; a ban on all sales or transfers of small arms and light weapons to the North; and the expulsion of North Korean diplomats who engage in "illicit activities."

North Korean citizens in the capital, Pyongyang, interviewed by The Associated Press said Thursday, in an echo of official propaganda, they believe their country can fight off any sanctions.

"No kind of sanctions will ever work on us, because we've lived under U.S. sanctions for more than half a century," said Pyongyang resident Song Hyo Il. "And in the future, we're going to build a powerful and prosperous country here, relying on our own development."

North Korean state media earlier warned that the imposition of new sanctions would be a "grave provocation" that shows "extreme" U.S. hostility against the country. It said the sanctions would not result in the country's collapse or prevent it from launching more rockets.

Recent commercial satellite imagery indicates new activity, possibly preparations for a rocket engine test, at a North Korean launch site where the country fired a long-range rocket on Feb. 6, according to an analysis by the North Korea-focused 38 North website.

In another development that will anger the North, South Korean and U.S. officials were set to begin formal talks Friday on deploying a sophisticated U.S. missile defence system in South Korea.

The deployment of the Terminal High-Altitude Area Defence, or THAAD, is opposed by North Korea, China and Russia. Opponents say the system could help U.S. radar spot missiles in other countries.

South Korean President Park Geun-hye said Thursday she'll co-operate with the international community to try to end North Korean "tyranny that suppress the freedom and human rights" of its own people. She recently warned of the North's collapse, and the country's parliament has passed a human rights bill that will enrage Pyongyang with criticism of the abysmal treatment of its citizens.

In January, North Korea conducted its fourth nuclear test, which it claimed was a hydrogen bomb. Last month, it put a satellite into orbit with a long-range rocket that the United Nations and others saw as a cover for a test of banned ballistic missile technology.